Interleukin-1 (IL-1.alpha. and IL-1.beta.) plays an important role in inflammation, acting locally and systemically to induce other proinflammatory cytokines, chemotactic factors, adhesion molecules, acute phase proteins, and fever (1). Animals which lack expression of IL-1 or IL-1 receptor have reduced inflammatory responses (2-5). Cellular responses to IL-1 are mediated by a cascade of intracellular signaling events including activation of the stress-activated MAP kinases, c-Jun N-terminal kinase (JNK) and p38, as well as transcription factors NF-.kappa.B (6). Upon IL-1 binding, IL-1 receptor type I forms a complex with IL-1 receptor accessory protein (IL-1R AcP) (7-9). IL-1 receptor associated serine/threonine kinase (IRAK), and a recently identified homologue IRAK-2, are rapidly recruited to this receptor complex via the adaptor protein MyD88 (10-13). IRAK becomes phosphorylated and subsequently interacts with TRAF6, a member of the TNF receptor associated factor (TRAF) family (13,14). TRAF6 has been implicated in activation of both JNK and NF-kB (14, 15). TRAF6 associates with NF-.kappa.B inducing kinase (NIK), a MAP 3 kinase related protein which is essential for TNF.alpha. and IL-1 mediated NF-.kappa.B activation but has no effect on the activation of JNK or p38 (15-17). NIK interacts with and may directly activate the recently identified I.kappa.B kinases (18-20). I.kappa.B kinases are responsible for activation of NF-.kappa.B via phosphorylation of its inhibitory partners, the I.kappa.B proteins, leading to their degradation by proteasomes (21).
IRAK and IRAK-2 are homologous to Pelle, a Drosophila protein kinase identified genetically to be important in dorsal-ventral pattern formation and in pathogen resistance (22, 23). Pelle is essential for the activation of Dorsal, an NF-.kappa.B like protein which is mediated by Toll, an IL-1 receptor homologue in Drosophila (24). The rapid IL-1 dependent association of IRAK and IRAK-2 with the IL-1 receptor complex and their homology to Pelle suggest that IRAK and/or IRAK-2 may serve important functions in initiating IL-1 signaling. However, the roles of IRAK and IRAK-2 in activation of the multiple downstream IL-1 signaling pathways have not previously been determined.
To dissect the role of IRAK in IL-1 signaling pathways, the IRAK gene was disrupted by homologous recombination and IRAK deficient fibroblasts were prepared. IL-1-induced activation of JNK, p38 and transcription factor NF-.kappa.B, and subsequent induction of IL-6 were analyzed in the IRAK-deficient cells.